Women with hereditary breast cancer risk have limited options for management. Close cancer screening or prophylactic mastectomy (PM) is most commonly utilized. Chemoprevention with tamoxifen is another option, but data regarding long-term risk reduction are incomplete and conflicting. Unfortunately, there are insufficient data regarding the expected outcomes with either surveillance or PM. Breast cancers on the chest wall have been documented following PM, developing in residual glandular epithelium. Thus, the extent of risk reduction with PM, if any, has been questioned. The program announcement "Clinical Epidemiology Studies in Hereditary Breast/Ovarian Cancer" describes the "immediate need to address these issues through retrospective studies based on existing resources". The Mayo Clinic has served as a referral center for specialized surgeries, including PM, for many years. They have access to two groups of high-risk women who elected PM: unaffected women who have elected contralateral PM and women treated with therapeutic mastectomy for their first breast cancer who elected contralateral, unilateral PM. With the work proposed, the investigators will clarify the magnitude of breast cancer risk reduction with PM using a combination of methods, including standardized morbidity ratios using the Gail and Anderson models to predict the expected number of events, and a relative risk using sisters of women who had PM as a reference group. We will also define short- and long-term complications following PM.